How Repetition Rate Impacts Detection Limits of Ion Mobility Spectrometers with Field-Switching Ion Shutters

Ion mobility spectrometers are extremely sensitive analytical instruments, consisting of an ionization region and a drift region, usually separated by an ion shutter. Highest sensitivity is reached with field-switching ion shutters, as ions are accumulated in the ionization region while the ion shutter is closed, which defines the reaction time in the ionization region. This study investigates the effect of repetition rate and reaction time on ion formation and detection limits. The results reveal that increasing the ionization source intensity and reaction time increases the signal-to-noise ratio even though less spectra can be averaged in a given overall measuring time. It is shown that the formation of protonated monomers and proton-bound dimers is considerably slower than the formation of reactant ions, highlighting the impact of reaction time on signal-to-noise ratio. At maximum ionization source intensity and optimal reaction times for the protonated monomers and proton-bound dimers of 1-butanol, limits of detection of 1.9 ppt_v and 110 ppt_v could be reached. For the protonated monomers and proton-bound dimers of 2-butanone, the limits of detection are 1.3 ppt_v and 57 ppt_v. The given limits of detection refer to an averaging time of 1 s. Although the optimal reaction times differ for different protonated monomers and proton-bound dimers, a reaction time of 40 ms was identified as a good compromise. These findings provide valuable insights into how the reaction time, and thus repetition rate and cycle time, impacts the detection limits of ion mobility spectrometers equipped with field-switching ion shutters.